The Wobbe index constitutes the principal magnitude which is characteristic of a gaseous fuel and, in that quality, it intervenes in the regulations of the combustion of gas burners.
Thus, all other things being the same, the heat flow rate of a burner is proportional to the Wobbe index, and its excess of air directly depends on it.
Since the networks for the transport and distribution of natural gas are becoming increasingly linked, and since they are supplied by a variety of energy sources, the Wobbe index of a gas of a given type can vary within non-negligible proportions, for example, ±5% at a given point of a network.
However, certain industrial methods, particularly in the glass and lime industries, are sensitive to these variations to the point that they require the use of specific combustion regulation solutions, one of these solutions consisting in incorporating the result of a local measurement of the Wobbe index in the regulation algorithms.
To date, all the apparatuses for measuring the Wobbe index which are available on the market are still relatively complex and consequently expensive.
Three principles are known to determine the Wobbe index.
The first principle consists in associating a measurement of the caloric power, obtained by calorimetry or chromatography, with a measurement of density, obtained by densitometry or by chromatography.
The second principle consists in analyzing the products of the combustion of the gas concerned in a small furnace where a sample of this gas is burned stoichiometrically or with an excess of air.
The third principle consists in measuring the physical characteristics of the gas, such as viscosity, heat capacity, etc., and in making a correlation between these measurements and the Wobbe index.
Two examples of the third principle are described in the patent documents DE-41 18 781 and U.S. Pat. No. 4,384,792.
The document DE-41 18 781 indeed describes a method whose purpose is to provide other measurements in addition to that of the Wobbe index, having recourse to two correlation functions with four constants, using three measurements of flow rate, one measurement of differential pressure and one measurement of temperature, and requiring a calibration with methane.
The document U.S. Pat. No. 4,384,792 describes a method for measuring the Wobbe index, having recourse to a correlation function with three constants, using a measurement of volume flow rate, a measurement of differential pressure and a measurement of temperature, and requiring a pressure regulator and a calibration by means of a hydrogenated gas.
These known techniques thus present a relatively high level of complexity, and the purpose of the invention is to propose a method for measuring the Wobbe index, which is easier to carry out, and in addition, a device which is substantially less complex and less expensive than the known devices.